If the scores are given for particular sessions then why are we calculating rank as a whole taking no. of students as 8,74,469?

Hello. The results are based on percentile score which is done for the first time in JEE MAINS 2019 exam. The percentile scores indicates the percentage of candidates that have scored equal to or below that particular percentile in that exam. Therefore the topper will get 100 percentile most probably. The marks obtained between the highest and lowest is also converted to appropriate percentile. This is done so that there is normalisation of the marks and no student is disadvantaged or benefitted from a particular question paper set or the session for which they appeared. The percentile scores for the total raw score for all the 4 sessions (in january exam) will be merged and that is your NTA score. This will be used for compilation of result and preparation of the Overall merit list after the April 2019 exam results.

The no. 874469 is an approximation of the total no. of students which may vary (obviously). This whole no. is taken so that students can have an approx idea of their rank.
For more information about how to convert percentile to rank check the below link.

http://www.entrance360.com/engineering/how-to-calculate-rank-for-jee-mains-2019/

If you want to know your probable rank then you can use our rank predictor below or you can comment down your score and I can give you an approximate rank range for your score.
http://www.engineering.careers360.com/jee-main-rank-predictor

Hope this helps. For any other questions feel free to comment down below.